1. Field of the Invention
The present invention relates to an automatic transmission fluid filter, and more particularly to an aesthetically pleasing automatic transmission fluid filter designed to be able to have improved case rigidity and fabrication properties as well as to be able to reduce pressure loss during use of the filter.
2. Description of Related Art
A commonly known conventional automatic transmission filter includes, for example as shown in FIG. 10, an upper case 102 having an outlet port 102a, a lower case 103 having an inlet port 103a, and a filter element 106 including a filter medium 104 and a holder frame 105 holding the periphery of the filter medium 104 and held between both cases 102 and 103. A plurality of ribs 107 are formed to extend in the width direction of the case on inner surfaces of both cases 102 and 103 so as to enhance the case rigidity. These ribs 107 prevent deformation of the case toward the filter element due to suction negative pressure and a consequent increase in pressure loss, which will lead to performance deterioration. Japanese Patent Application Laid-Open No. 8-24532 discloses a fluid filter with an upper case and a lower case formed with ribs on their inner surfaces for preventing deformation of the filter medium.
However, with the conventional automatic transmission fluid filter mentioned above, sink marks easily occur during the case molding process because of the plurality of internally formed ribs 107 of the cases 102 and 103, resulting in poor outer appearance. The case appearance is significantly deteriorated, in particular, when a case that is transparent or translucent is joined with the holder frame by laser welding. The ribs also cause an increase in the amount of use of resin which leads to a higher production cost. Another problem was that the ribs have a large impact on the flow properties of a fluid flowing through the filter chamber during use, causing an increase in pressure loss.